1. Field of the Invention
The present invention generally relates to the electronics industry and, more specifically, to radiofrequency transceiver systems. The present invention more specifically relates to a directional coupler and applications thereof.
2. Discussion of the Related Art
A coupler is generally used to sample part of the power present on a so-called main or primary transmission line, with respect to another so-called coupled or secondary line, located nearby.
Couplers can be classified in two categories according to whether they are formed of discrete passive components (couplers with lumped elements) or of conductive lines arranged close to one another to be coupled (distributed couplers). The present invention relates to the second category of couplers.
In many applications, it is needed to sample part of the power transmitted over a line, for example, to control the power of an amplifier in a transmit circuit, to control the linearity of a transmit amplifier according to the losses linked to the reflection of an antenna, to dynamically match an antenna, etc.
A coupler is defined, among others, by its directivity which represents the power difference (expressed in dB) between the two access ports of its coupled or secondary line. Theoretically, an ideal coupler has an infinite directivity, that is, no power is present on the port of its secondary line located opposite to the output port of its main line when a signal runs through this main line from the input port to this output port. In practice, a coupler is said to be directional when its directivity is sufficient (typically greater than +20 dB) for the powers recovered from the access ports of its secondary line to enable to make out the direction of the power flow in its main line. When the two ports of the secondary line of the coupler can be used to simultaneously have the power information, the coupler is said to be bidirectional. In this case, the respective input and output ports of the main line and of the secondary line may be inverted.
If all ports are perfectly matched (typically, at 50 ohms), no stray reflection occurs and the coupler operates ideally. Such a perfect matching can unfortunately not be obtained in practice. In particular, the output port (typically, to which an antenna is connected) may undergo impedance modifications even in real time under the effect of modifications in the environment of the antenna. Such modifications generate stray reflections, which results in return loss, to be taken into account in the transmission chain.
A lack of directivity of the coupler adversely affects the accuracy of the measurements of a mismatch of the main line output port. Now, this mismatch is an important criterion of the transmission. The return loss is assessed on one of the ports of the secondary line of the coupler. Its measurement is, for example, used to modify the parameters of an impedance matching network interposed between the main coupler line and the antenna.
The signal sampled from the secondary line is tainted with non-negligible errors and is no longer usable when the coupler directivity is lower than 20 dB. The output impedance of the coupler can then no longer be controlled, whereby the return loss cannot be corrected.
To overcome a possible mismatch of the port of the secondary line of the coupler from which the data are sampled, the ends of the secondary line are sometimes equipped with attenuators. Such attenuators have no effect on the actual directivity of the coupler.